You cannot disconnect the immobiliser as it is the ECU, that reads the chip from the old key that you have lost.
If you find that key take the key fob apart, you will find a little black chip inside.
Swap it for the one that fits your ignition barrel it should start.
Failing that you can use your existing chip in the key you have got for the barrel and take it to Vauxhall, or someone who can program the chip to read your ECU.
Its a T**t if you have to change the barrel, but I hope this helps.
Corsair, February 2008
